The present invention relates to a power circuit that supplies DC excitation to a gas discharge lamp. More particularly, the present invention relates to a ballast circuit that automatically and sequentially provides desired transfer functions for the DC operation of the metal halide lamp.
The ballast circuit having its desired transfer functions provides the necessary voltage and current conditions to the gas discharge lamp, such as a metal halide lamp, so that it properly performs during its various modes of operation which are; (1) an initial high voltage breakdown or start mode; (2) a glow-to-arc transition or glow mode; and (3) a steady state, arc or run mode. Each of these modes have different voltage and current requirements allowing the gas discharge lamp to perform in a proper manner. It is desired that these various current and voltage requirements be provided in an automatic and sequential manner so that the lamp transitions from its start, to its glow, and then to its run mode. Similarly, it is desired that these voltage and current conditions be automatically provided if the lamp reverts from its run mode to its glow or start mode.
The current developed by the ballast circuit which flows through the gas discharge lamp may typically experience surges as these lamps enter or revert from its modes of operation, for example, as the lamp ionizes and enters into the glow mode, the current flowing at this entrance is of a relatively large value and is of a relatively high rate of change which may be such so as to detrimentally affect the associated circuit components of ballast circuit and especially the lamp itself. It is desired that a ballast circuit be provided with means to limit the maximum amount of current supplied to the lamp during its modes of operation and also to limit the related maximum rate of change of the current.
Ballast circuits commonly comprise impedance devices that are utilized for supplying the necessary conditions for operating a gas discharge lamp. These impedance devices in ballast circuit operated from a typical AC power source create an undesirable condition with regard to lowering the related power factor which, in turn, causes a leading or lagging current relative to the AC power source. The resulting low power factor becomes a concern to the lighting installation designers who must size the conductors and to the electric power companies who must provide the additional reactive power. It is desired that a ballast circuit be provided with means that are easily adapted to present a high power factor to a power source of not less than 0.90. It is especially desired to present this high power factor during the steady-state or run mode of the lamp.
Accordingly, it is an object of the present invention to provide a ballast circuit having means for easily adapting and presenting a high power factor to the AC power source supplying the gas discharge lamp.
It is a further object of the present invention to provide a ballast circuit that presents the desired current and voltage conditions to the gas discharge lamp, in an automatic manner, so that the lamp is allowed to transition to or revert from its start, glow or run modes of operation while still yielding proper lamp performance.
It is a further object of the present invention to provide a ballast circuit having means that limits the amount of current and the rate-of-change of the current supplied to the lamp during its start, glow and run modes of operation.